1. Field of the Invention
The invention relates to the field of optical tools and methods for adjusting the lead-lag phasing of helicopter rotor blades.
2. Description of the Prior Art
A rotor system in a helicopter includes a rotor hub drive, a plurality of blades, and a support assembly to which the helicopter blades are attached by a suitable connecting means, such as a rotor blade strap pack which connects the blades to the rotor hub, which permits flapping of the blades as well as changes in pitch (feathering), and which permits changes in the lead-lag angle. Typical prior art helicopter rotor systems are comprised of multiple blades. Lead-lag adjustment means is provided for changing the degree by which the tip of a helicopter blade angularly deviates from the straight line which is perpendicular to the instantaneous direction of blade motion. Generally, the straight line representing a zero lead-lag adjustment is that line which includes the aerodynamic center of the blade tip; the hinge pin, which is near where the root of the blade is coupled to the strap pack; and the central point of the rotor hub. If any one of the blades should have a lead-lag adjustment different than others, a substantial stress occurs upon the rotor system and helicopter in general.
The prior art alignment methods for multiple blades included methods as simple as drawing a string under tension from the center of the rotor hub to the blade tip in order to compare the angle of the string made to the zero lead-lag line as evidenced by the displacement of the string from the hinge pin.
Other prior art methods include the use of a low power telescope mounted at the central point of the rotor hub to sight toward the blade tip and to measure the angular deviation between the blade tip and the hinge point.
However, both methods are subject to errors due to the flexibility of the helicopter blade. Thus, when such a test was statically made, the blade tip would droop toward the ground making the measurement difficult and in most instances the blade tip would droop to position which was either forward or behind the position the blade would assume during operation. In addition thereto, helicopter blades are designed to have an inherent twist along the length of the blade. When the blade droops in the static position, this twist will cause displacement of the blade tip from the position the blade will assume when it is straightened under centrifugally loaded operation. After the lead-lag angular adjustments had been "eye-balled" by a prior art method, the only means for testing the adjustment is to test fly the helicopter and measure the direction and magnitude of the remaining vibration. Such in-flight accelerometers, used to measure the direction and magnitude of the vibration, are well known to the art and heretofore have been relied upon as the principal means by which the lead-lag angles could be practically adjusted.
However, it is no longer convenient to practice the prior art methods for lead-lag adjustment inasmuch as modern helicopters often incorporate a substantial amount of equipment within or on the hub, such as radar equipment, air-speed data measuring devices and deicing equipment, all of which make the use of hub-mounted transits virtually impossible. The alignment of rotor blades during military operations is further aggravated in that alignment at night by these prior art methods is extremely difficult and haphazard due to the lack of necessary lighting to accurately practice the prior art methodology.
What is needed then is a method and apparatus for optically phasing the rotor blades which will overcome each of the limitations and disadvantages of the prior art methods and apparatus. A means must be devised which provides optical blade phasing in helicopters which is easy to install and to set up, and which avoids conflict with hub mounted devices such as air-speed indicators, radar installations and deicing equipment.